Yes, sometimes a person is on the surface materialistic, but may be suffering the repercussions of emotional trauma through a fractured home life and neglectful parenting.
Their comfort is receiving something (money, possessions) that they perceive is an outward sign of love and caring. Other times they act irrationally, out of unresolved hurt and loss.
are you able to say how much the 25% will be?
Irrespective of how much the inheritance is, you have choice and options -
you could give him 25% and he's unhappy with it even if it's a meaningful amount and he goes out of your life in a huff because he feels hard-done by.
you could give him 50% and off he goes out of your life anyway, because he's mixed up, sad, bitter about how life has treated him.
you could give him all the inheritance and he still does a disappearing act.
you can't buy love, so no matter what you decide to give him, prepare yourself for the worst, that he walks away with some £££ or no £££ because actually it sounds like what he needs is therapy not money.